Method and device for solid phase microextraction and desorption

ABSTRACT

A device for carrying out solid phase microextraction is a fiber contained in a syringe. When it is desired to analyze a sample in a bottle having a septum, the needle is inserted through the septum and the plunger is depressed so that the fiber will extend into the sample. After one or two minutes, the plunger is moved to the withdrawn position so that the fiber will return to the needle and the syringe is withdrawn from the sample bottle. The syringe is then inserted through a septum in a gas injection port of a gas chromatograph. The plunger is again depressed so that the fiber will extend into the gas chromatograph and an analysis of the components on the fiber is carried out. Then, the plunger is moved to the withdrawn position and the syringe is withdrawn from the injection port.

This is a continuation application of application Ser. No. 08/826,682filed Apr. 7, 1997 now abandoned, which is a continuation application ofapplication Ser. No. 08/306,435 filed Sep. 19, 1994 now U.S. Pat. No.5,691,206 which is a continuation-in-part Application of applicationSer. No. 07/934,736 filed Oct. 10, 1992 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and device for solid phasemicroextraction and analysis and, in particular, relates tomicroextraction and analysis being carried out using various types of asingle fiber which can be coated with various materials or uncoated.

2. Description of the Prior Art

Presently, in the organic analysis of environmental samples whichinvolve the separation of components of interest from such matrices assoil, water, fly ash, tissue or other material, liquid extraction istradionally used as the separation process. For example, water samplesare usually extracted with organic solvent. Similarly, solid samples areleeched with an organic solvent in a SOXHLET apparatus. Methods based onsolvent extraction are often time consuming, difficult to automate andare very expensive since they require high purity organic solvents andthese organic solvents are expensive to dispose of. Further, the organicsolids usually have high toxicity and are difficult, to work with. Inaddition, the extraction processes can be highly non-selective.Therefore, sequential chromatographic techniques must sometimes be usedto separate complex mixtures after extraction, significantly increasingthe overall analysis time and the cost. EP-A1-159 230 discloses anextraction method of components in a liquid by placing packets of fibersin contact with said liquid in extracting the components.

Solid phase extraction is a known effective alternative to liquid-liquidextraction in the analysis aqueous samples. The primary advantageof-solid phase extraction is the reduced consumption of high puritysolvents and the resulting reduction in laboratory costs and the costsof solvent disposal. Solid-phase extraction also reduces the timerequired to isolate the analyte of interest. However, solid phaseextraction continues to use solvents and often suffers from high blankvalues. Further, there is considerable variation between the productsoffered by different manufacturers and lot-to-lot variation can be aproblem when carrying out solid phase extraction procedures. Solid phaseextraction cartridges available for manufacturers are normallyconstructed of plastic which can adsorb the analyte and increaseinterferences in the analysis. The disposable plastic cartridges used inthe solid phase extraction process are first activated using organicsolvent. The excess organic solvent is then removed and the sample to betested is passed through the cartridge. The organic components from thesample. are adsorbed on the chemically modified silica surface of thematerial in the cartridge. Both molecules of interest as well asinterferences are retained on the cartridge material. During desorption,a selective solvent is chosen to first remove the interferences. Theanalyte is then washed out of the cartridge. The analytical procedurefrom that point is identical to that used in liquid-liquid extraction.The analyte is first preconcentrated and the mixture is then injectedinto an appropriate high resolution chromatographic instrument. Stepsinvolving the use of organic solvents are the most time consuming.

SUMMARY OF THE INVENTION

A device for carrying out solid phase microextraction of componentscontained in a fluid carrier is characterized by, in combination, afiber and a housing surrounding said fiber, said housing containingaccess means so that said carrier and components could be brought intocontact with said fiber.

A method of carrying out solid phase microextraction and analysis withcomponents contained in a carrier uses a fiber. The method ischaracterized by placing said fiber in contact with said carriercontaining said components for a sufficient period of time for chemicalextraction to occur, subsequently removing said fiber from said carrierand placing the fiber into a suitable analytical instrument and carryingout desorption with respect to at least one component on said fiber.

A method of carrying out solid phase microextraction and analysis withcomponents contained in a carrier uses a fiber contained in a housing.The housing has access means so that said carrier can be brought intocontact with said fiber. The method is characterized by contacting saidfiber with said housing for a sufficient time to allow chemicalextraction to occur, ending said contact and placing said fiber in asuitable analytical instrument in such a manner that desorption occurswith respect to at least one component on said fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional side view of a syringe and fiber with theplunger depressed;

FIG. 2 is a schematic side view of a slightly different syringe andfiber with the plunger withdrawn;

FIG. 3 is a schematic side view of a needle portion of a syringecontaining a hollow fiber;

FIG. 4 is a graph of amount of analyte extracted versus time;

FIG. 5 is a graph showing the results of a typical gas chromatographyanalysis;

FIG. 6 is a graph showing another analysis from a gas chromatograph;

FIG. 7a shows a chromatogram produced when using the solid phasemicroextraction of the present invention;

FIG. 7b shows a chromatogram produced when using the prior art method ofliquid-liquid extraction for the same components as those of FIG. 7a;

FIG. 8 is a chromatogram of the extraction of gasoline components fromwater with silicone coated fibers; and

FIG. 9 is a chromatogram from the extraction of organics from coalgasification waste water using a silicone coated fiber.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2 in greater detail, a device 2 for carryingout solid phase microextraction has a syringe 4 containing a fiber 6.The syringe 4 is made up of a barrel 8 which contains a plunger 10 andis slidable within the barrel 8. The plunger 10 has a handle 12extending from one end 14 of the barrel 8. At the opposite end 16 of thebarrel 8, there is located a needle 18 which is connected to the end 16by the connector 20. The handle 12 and the needle 18 and connector 20are shown in an exploded position relative to the barrel 8 for ease ofillustration.

The fiber 6 is a solid thread-like material that extends from the needle18 through the barrel 8 and out the end 14. An end of the fiber 6 (notshown) located adjacent to the cap 12 has retention means 22 locatedthereon so that the fiber will move longitudinally as the plunger 10slides within the barrel 8. The retention means can be simply a drop ofepoxy which is placed on the end of the fiber 6 near the handle 8 andallowed to harden. The fiber 6 is partially enclosed in a metal sleeve24 which surrounds that portion of the fiber 6 located within theplunger 10, the barrel 8 and part of the needle 18. The purpose of themetal sleeve 24 is to protect the fiber 6 from damage and to ensure agood seal during operation of the device. Extending from the connector20 is an optional inlet 26. The purpose of the inlet 26 is to allowalternate access to the fiber. For example, when the fiber is containedwithin the needle 18, fluid could contact the fiber 6 by entering theinlet 26 and exiting from a free end 28 of the needle 18. The inlet 26can also be used to contact the fiber with an activating solvent.

In FIG. 2, a schematic version of the device 2 is shown. The plunger isin a withdrawn position and the free end of the fiber 6 is locatedentirely within the needle 18. The access permitted by the inlet 26 whenthe fiber is in the position shown in FIG. 2 can readily be understood.Obviously, fluid contacting the fiber 6 within the needle 18 could alsoenter the free end 28 of the needle 18 and exit from the access 26.

In FIG. 3, only the needle portion of the device is shown. A fiber 30extending from the metal sleeve 24 is hollow. It can be seen that thereis an opening 32 in the wall of the metal sleeve 24 to allow access toan interior of the sleeve 24 as well as an interior of the fiber 30. Forexample, fluid could enter the inlet 26 and an interior of the needle18. Then, the fluid could pass through the opening 32 and through aninterior of the fiber 30 and ultimately exit from the free end 28 of theneedle 18. In this embodiment, the fiber does not extend to the handle12 (not shown) but only the metal sleeve 24 extends to the handle 12.The fiber 30 can still be moved beyond the end 28 of the needle 18 bydepressing the plunger and returned to the position shown in FIG. 3 bymoving the plunger to the withdrawn position.

Alternatively, if it is desired to have the fiber 30 located within theneedle 18 at all times, contact with the fiber 30 can be attainedthrough the inlet 26 or the opening 32 and the free end 28. A plug 33located within the metal sleeve 24 prevents any fluid from travelling upthe sleeve to the handle. In some situations, the fluid could flowthrough the sleeve 24.

In general terms, the syringe could be said to be a housing for thefibers 6, 30 and the access means could be the action of the plunger 10in moving the fiber beyond the end 28 or, alternatively, the accessmeans could be the inlet 26.

The disadvantages and inconveniences of the previous processes foranalyzing various fluids are overcome by the solid phase microextractiontechnique of the present invention. The diameter of the fibers will varybut will preferably be between 0.05 millimeters and 1 millimeter. Muchof the experimentation on which the present invention was based, wascarried out using fused silica fibers that were chemically modified. Thefused silica fibers are widely used in optical communication and areoften referred to as optical fibers.

Chemical modification of these fibers can be achieved by the preparationof the surface involving etching procedures to increase the surface areafollowed by chemical attachment of the desired coating. The stationaryphases bonded to the surface of the silica fibers are similar to thatused in fused silica gas chromatograph columns or high performanceliquid chromatography columns.

As an example, fused silica fibers were obtained from PolymicroTechnologies Inc., Phoenix, Ariz. and these fibers were coated withpolyimide and had an outer diameter of approximately 171 μm. Uncoatedfused silica was obtained by burning off the polyimide coating andgently scraping off the charred portion. To use the polyimide film as astationary phase, it was first heated at 350° C. for four hours. Thepolyimide was then burned off and the char removed, except for a one totwo millimeter portion at the end of the fiber. In all cases, thepolyimide was burned off after the fiber had been inserted into thesyringe and trimmed to the correct length. After burning, the fiberbecame fragile and had to be handled carefully. The metal casing is usedto strengthen the fiber. The normal lifetime for a prepared fiber wasfive to six weeks with regular use.

The solid phase microextraction process does not require a sophisticatedcoating system to be a useful technique. Either the uncoated fiber,fused silica, silicone or the polyimide films that optical fibers areshipped with can be a suitable stationary phase.

The method of solid phase microextraction and analysis consists of a fewsimple steps. For example, when a water matrix sample containingcomponents of interest is desired to be analyzed, the plunger of thesyringe is depressed and the exposed fiber extending from the free endof the needle is inserted into the water matrix sample. The organiccomponents of the water are extracted into the non-polar phase. Water isconsidered to be the carrier in a water matrix sample. Where the watersample is contained in a bottle containing a septum, the needle isinserted through the septum first before the plunger is depressed sothat the fiber will not be damaged by the septum. When themicroextraction has occurred to a sufficient degree (usuallyapproximately two minutes), the plunger is moved to the withdrawnposition causing the fiber to be drawn into the needle and the needle isremoved from the sample bottle through the septum. Preferably, thesample is stirred while the fiber is inserted. The time for extractionwill depend on many factors including the components being extracted aswell as the thickness and type of coating, if any, on the fiber.Usually, the extraction time is approximately two minutes. The plungeris then moved to the withdrawn position to retract the fiber into theneedle. The needle is then removed from the bottle and is insertedthrough the septum in an injection port of a conventional gaschromatograph or other suitable analytical instrument. The plunger isthen depressed again to expose the fiber and the organic analytes on thefiber are thermally desorbed and analyzed. The fiber remains in theanalytical instrument during the analysis. When the analysis has beencompleted, the plunger is moved to the withdrawn position and thesyringe is removed from the injection port. Various injection ports aresuitable such as the “split-splitless” type or the “on-column” type.

While various types of syringes will be suitable, a HAMILTON 7000 (atrade mark) series syringe has been found to be suitable. The syringefacilitates convenient operation of the solid phase microextractionprocess and protects the fiber from damage during the introduction intoa sample bottle or into an injector of an analytical instrument or evenduring storage. The length of the fiber depends on the injector of theanalytical instrument with which the fiber will be used. Preferably, thefiber is mounted in a housing to a movable part so that the fiber ismovable longitudinally within the housing. Still more preferably, themovable part moves a sufficient distance so that at least part of saidfiber can be extended outside of said housing and retracted into saidhousing successively. The movable part is preferably an elongated memberwhich extends partially outside of the housing. The part of theelongated member that extends partially outside of the housingpreferably has a handle thereon. The elongated member can be a plunger.

In addition to the improved convenience of the present device andmethod, the method differs significantly in the extraction part of theprocess compared to the prior art solid phase extraction process usingcartridges. The extraction process in accordance with the presentinvention does not require prior sampling of aqueous material sincein-vivo or in-vitro sampling can be conveniently performed. Themicroextractor can be directly inserted into the fluid stream. Thesimple geometry of the fiber eliminates clogging caused by particlematter present in the samples. Also, due to the small size of the fiber,not all of the organic compounds are extracted but rather theequilibrium described by the partition coefficient between the water andorganic stationary phase for a given analyte is established. Therefore,the solid phase microextraction method of the present invention can bemade selective by appropriate choice of a specifically designed organicphase. The partitioning between the aqueous phase and the organiccoating can be described through the distribution constant, K:$\begin{matrix}{K = \frac{C_{s}}{C_{aq}}} & (1)\end{matrix}$

where C_(s) is the concentration in the stationary phase and C_(aq) isthe concentration in the water. The partition ratio, k′, is therefore:$\begin{matrix}{k^{\prime} = {\frac{C_{s}V_{s}}{C_{aq}V_{aq}} = {\frac{n_{s}}{n_{aq}} = {K\frac{V_{s}}{V_{aq}}}}}} & (2)\end{matrix}$

where n_(s) and n_(aq) are the number of moles in the stationary andaqueous phases, respectively, and V_(s) and V_(aq) are the volumes ofthe respective phases.

Rearranging Eqn. 2 yields: $\begin{matrix}{{ns} = {K\frac{V_{s}n_{aq}}{V_{aq}}}} & (3)\end{matrix}$

substituting C_(aq)V_(aq) for n_(aq) results in:

n _(s) =KV _(s) C _(aq) =AC _(aq)  (4)

where A=KV_(s).

A linear relationship between concentration of analytes in aqueoussamples and detector response is expected based upon the relationship inequation (4). The slope of the linearity curve can be used to determinethe partition coefficient for a given analyte if the volume of thestationary phase is known. Furthermore, the sensitivity of the fiber canbe adjusted by changing the volume (thickness or area) of the stationaryphase.

The linear dynamic range of the method typically extends several ordersof magnitude for coatings similar to chromatographic stationary phasematerials. The limit of quantization depends on the partitioncoefficient and the thickness of the coating and can be as low as a fewppT (parts per trillion), which was obtained for chlorinated solvents.In this case the amount of the solvents extracted by a thick polyimidecoating from a water sample is about 30 pg per component at a 1 μg/Lconcentration. This amount ensures not only ECD detection but will allowmass spectrometric identification and quantization.

The dynamics of the extraction process is illustrated on FIG. 4 whichshows an example of a typical relationship between the amount of analyteadsorbed onto the microextractor (peak area) versus the extraction time,which corresponds to the exposure time of the fiber to the water matrixsample. Initially, the amount of analyte adsorbed by the stationaryphase increases with the increase in extraction time. This trend iscontinued until the point of steady state is achieved which causes therelationship to level off. This situation indicates the state ofequilibrium between the concentration of the analyte in the stationaryphase and in the water matrix sample and defines optimum extractiontime. According to FIG. 4, optimum extraction time for uncoated fiber(about 0.1 μm film of silica gel) and PCBs as analytes is -about oneminute.

FIG. 5 illustrates the chromatogram corresponding to a PCB mixture inwater extracted and analyzed by the solid phase microextraction method.Peak tailing is larger for the more volatile compounds than the heavier,later eluting components. This is an effect of thermal focussing thatoccurs when the analytes are volatilized at 300° C. and transferred to a150° C. oven. The heavier compounds benefit from thermal focussing, butthe oven is at too high a temperature to allow focussing of the morevolatile compounds. The tailing can be alleviated by using acryogenically cooled oven to improve focussing.

An uncoated fiber can also be used to adsorb benzene, toluene, ethylbenzene and xylenes (BTEX) from aqueous solutions. For this separation(FIG. 6), a flame ionization detector (FID) was used, illustrating thata sufficient quantity was adsorbed for FID detection. This expands thegeneral applicability of the fiber as FID detectors are somewhat easierto operate and maintain than ECD detectors. The extraction efficiency inthis case is sufficiently high to deplete significantly the analyteafter 2 to 3 injections if a small volume of aqueous material (1 to 2mL) is sampled. A larger sample volume (100 mL) is thus recommended ifmultiple injections are necessary.

Moderate levels of organic interferences and variation in ionic strengthof aqueous solution do not significantly change the extractionequilibria. However, large amounts of organic solvent could be addedintentionally to introduce partitioning selectivity, as is commonly donein liquid chromatography.

The fiber method has great potential for the analysis of highly sorptivecompounds that can be difficult to sample without loss of analyte.Losses to storage bottles and transfer lines could potentially beeliminated by sampling in situ and analyzing the fiber in the fieldusing portable gas chromatograph instrumentation. The device and methodof the present invention can utilize a mechanical device such as anautosampler. The autosampler can be programmed to operate the plunger atthe appropriate time to contact the carrier and to insert the syringeand the fiber into the injection port of the analytical instrument. Theautosampler has an advantage over manual extraction and analysis in thatthe contact time and the length of the fiber in the carrier as well asin the instrument can be maintained constant. A VARIAN 3500 gaschromatograph and a VARIAN 8100 autosampler has been found to besuitable.

Possible applications of this technique include sampling of both surfaceand groundwater samples, either in situ or in the laboratory. It couldpotentially be used in on-line process applications or clinicalanalysis. Both of these applications benefit from the simplified samplepreparation. The coating can be designed for either a broad scan of theorganic contaminants (non-selective fiber coating) or selectivesampling. This method, when combined with laser desorption, could reducethe sample extraction and analysis to a fraction of a minute. In thistechnique the optical fiber is used as alight guide. In a variation ofthe invention, the syringe could have a laser source affixed theretowith activation means and coupling optics to focus light onto the fiberwhich will transmit the light to a free end thereof to desorb thecomponents thereon. Curie point heating and microwave desorption arealternative desorption methods. The fiber also shows promise as a methodof studying the adsorption properties of polymers and for obtaininginformation about partitioning in liquid chromatographic systems.

FIG. 7 illustrates the advantages of the method of the present inventioncompared to the prior art solvent procedure. The chromatogram from FIG.7a corresponds to silica fiber techniques using C-18 coating and FIG. 7bto liquid-liquid extraction with chloroform. In both cases the sameeffluent from a sewage treatment plant was analyzed under the samechromatographic conditions. Results are similar, however the totalextraction time was about an hour for the solvent method and two minutesfor the fused silica fiber technique. The chromatogram for FIG. 7b showsthe presence of the solvents used in the liquid-liquid extraction. Thesolid phase microextraction device facilitates easy sampling in thefield. In addition, when organic solvents are used in the preparationstep, the corresponding large peak together with possible impurities canmask volatile analytes (FIG. 7b).

In FIG. 8, there is shown a chromatograph for the extraction of gasolinecomponents from water using a silicone coated fiber. In FIG. 9, there isshown a chromatograph for the extraction of organics from coalgasification waste water using a silicone coated fiber. Both analysesand identifications for FIGS. 8 and 9 have been done using a massspectrometry detector.

The device and method of the present invention can also be used forextraction and analysis of gases and for supercritical fluids as well.The method is not limited to analysis of organic analytes but also forinorganic ions by using ion-exchange materials located on the fibersurface. In addition to thermal desorption by direct heating, laserdesorption or conductive heating, for example, microwave desorption orCurie point magnetic hysteresis method could be used. Various fiberswill be suitable depending on the use that is being made of the presentinvention. For example, fused silica, graphite fibers, fibersconstructed with solid polymeric materials and even metal wires can beused as fibers and the fibers can be coated with various materials oruncoated. Some suggested coatings are CARBOWAX (a trade mark),octadecyltrichlorosilane, polymethylvinylchlorosilane, liquidcrystalline polyacrylates, silicone, polyimide and graftedself-assembled monolayers. Fibers coated with these coatings,are storedunder nitrogen or helium to prevent absorption of the volatile organicspresent in air. The coatings can be organic or inorganic, for example,fused silica surface.

In addition to having coating located on an outer surface of a solidfiber, coating could be located on an inner surface of a hollow fiber.Coating could also be located on the packing material used with thefiber. In addition to direct extraction, the method of the presentapplication could be performed with prior activation using organicsolvents by using the optional inlet 26 on the syringe. The analyticalinstrument used with the method of the present invention can also bevaried. For example, a gas chromatograph, a liquid chromatograph or asupercritical fluid chromatograph could be used. Other analyticalmethods such as flow injection analysis, mass spectrometry, atomicabsorption or emission including inductively coupled plasma techniquecould be used.

In addition to analyzing for environmental contaminants, the method anddevice of the present invention can be used to monitor or measure thecomponents in industrial process streams. The present invention can alsobe used to study properties of coatings, for example, absorption,deterioration rates and diffusion coefficients.

Numerous other variations, within the scope of the attached claims, willbe readily apparent to those skilled in the art.

I claim:
 1. A method of carrying out solid phase microextraction ofcomponents contained in a fluid carrier using a hollow needle with asurface disposed within the needle and an extracting phase coating onsaid surface, said coated surface defining a tubular passageway, amember supporting said needle at one end thereof, said member supportingsaid needle having a passageway, said needle having a free end, saidmethod comprising inserting an end of said needle into a fluid carrierto allow said carrier to contact with the extracting phase coating for asufficient time to allow selective microextraction to occur between saidfluid carrier and said coating, subsequently removing the needle fromthe fluid carrier and inserting the needle through an inlet of ananalysis instrument and desorbing components from the coating into theinstrument, there being no pre-extraction required of said componentsprior to entering said needle.
 2. A method as claimed in claim 1 whereinthe needle is placed in contact with the fluid carrier until thecomponents of the carrier reach equilibrium with the extracting phasecoating.
 3. A method as claimed in claim 1 wherein said stationary phasehas a predetermined volume to provide a redetermined sensitivity of saiddevice for extraction of an analyte from a fluid sample.
 4. A method asclaimed in claim 1 including the step of selecting the nature and volumeof said stationary phase to provide a dynamic range extending over atleast three orders of magnitude of analyte concentration.
 5. A method asclaimed in claim 1 wherein said carrier is a liquid carrier.
 6. A methodof carrying out solid phase microextraction of components contained in acarrier using a hollow needle with a surface disposed within the needleand an extracting phase coating on said surface, said coated surfacedefining a tubular passageway, said coating being effective forselective extraction of an analyte from a carrier fluid, a membersupporting said needle at one end thereof, said member supporting saidneedle having a passageway, said needle having a free end, said methodcomprising: (a) inserting an end of said needle into a carrier to allowsaid carrier to contact the extracting phase coating for a timesufficient to allow microextraction to occur between said carrier andsaid coating; subsequently removing the needle from said carrier; and(b) inserting the needle through an inlet of an analysis instrument anddesorbing components from the coating into the instrument, there beingno pre-extraction required of said components prior to entering saidneedle.
 7. A method as claimed in claim 6 wherein the carrier is aliquid carrier.
 8. A method as claimed in any one of claim 3, 4, or 6wherein said fluid carrier flows through the needle in contact with saidextracting phase.